1. Field of the Invention
The present invention relates to position sensors that utilize a magnetostrictive waveguide to implement a linear, curved, or rotary measurement path, while measuring the position of a permanent magnet target, called the position magnet. Multiple position measurements, or measuring the position over a time period, may also be used to provide a measurement of acceleration. The higher signal to noise (S/N) ratio of an improved sensor enables it to achieve a higher performance level, which may include reducing the level of error, reducing the power requirement, optimizing magnetic field strengths, or other parameters. The sensor improvements include a shielded waveguide, waveguide clamp, voltage selector, switched voltage increasing circuit, signal clamp, voltage adjusting circuit, adjustable switched capacitor filter, and a signal path that does not run through a microcontroller.
2. Description of the Prior Art
Magnetostrictive position sensors are well known in the Prior Art. Such Prior Art sensors are often designed for measuring linear position, but some curved and rotary versions have also been available on the market. In a magnetostrictive position sensor, a pulse of current (called the interrogation pulse) is applied to a waveguide (i.e. it is interrogated). The amount of current is usually in the range of 0.5 to 5.0 amperes, and the pulse duration is usually in the range of 0.5 to 5.0 microseconds. The pulse is repeated upon command or at a fixed or variable rate. When at a fixed rate, the amount of time between pulses is usually in the range of 0.1 microseconds to 0.1 seconds. A position magnet is located somewhere along the length of the waveguide, at a position that is to be measured. Upon application of the interrogation pulse, the waveguide becomes magnetized due to the current flow in the waveguide. Vector summing of the waveguide magnetic field with the field of the position magnet results in the application of a torsional force to the waveguide at the location of the position magnet. A torsional strain wave is formed, which travels along the length of the waveguide at a speed of approximately 3,000 meters per second. A pickup device for detecting the strain wave is mounted near one end of the waveguide, usually near the electronic circuitry needed to operate the sensor. When the interrogation pulse is applied, a timer is started. When the strain wave is detected by the pickup, the timer is stopped. The elapsed time measured by the timer is proportional to the distance between the position magnet and the pickup. So, the location of the position magnet can be determined with respect to the pickup. It is common practice in such a position sensor to connect a copper wire to one end of the waveguide and position the copper wire in parallel with the waveguide to provide a return path for the interrogation current. The copper wire is called the return wire or return conductor. It is also common practice to enclose both the waveguide and return wire within an outer metal tube. In this configuration, the current pulse is conducted through the waveguide, and back through the return wire, completing the circuit. But since the interrogation pulse can have an amplitude of several amperes and is applied to a long ferromagnetic material (the waveguide), electromagnetic interference (EMI) can be formed.
It is desirable to obtain a high signal to noise ratio in the measurement transducer as well as in the signal processing electronics. There are several prior art methods for increasing the signal level, as well as for reducing the noise level.
The signal level can be increased by increasing the current of the interrogation pulse, but excessive current in the interrogation pulse causes an increase in the power requirement of the position sensor. The signal level can also be increased by adding more turns to a pickup coil, but using very fine wire becomes a manufacturing difficulty, and using a pickup of a much larger physical size results in a signal pulse that is not as sharp, and therefore increases noise. Signal level can also be increased by using a waveguide material having higher permeability, but this sacrifices other desired qualities of the waveguide material, such as maintaining a uniform speed of the strain wave over a wide temperature range.
To maintain a high signal to noise ratio, it is important to keep a signal at a proper level for reliable detection. In the linear position sensor of U.S. Pat. No. 5,640,109, a method is taught for electronically adjusting the amplitude of the signal. Accordingly, the circuitry includes an adjustable first threshold value, a stored second threshold value, and a magnitude comparator that is used to adjust the second stored threshold value depending on whether or not the amplitude of the signal exceeds the first threshold value.